A magneto-optic disk unit must clear a target track before writing data to the target track. Namely, it must carry out an erase operation before a write operation on the target track. Since it takes a long time if the erase and write operations are done separately, a recent magneto-optic disk unit emits an erase beam and a write beam in respective vicinities of a target track, to carry out the write operation immediately after the erase operation.
To realize such a simultaneous operation, the disk unit has an optical head which is moved across tracks by a VCM positioner, two lens actuators (track actuators) disposed on the optical head, and object lenses arranged on the lens actuators to pass the erase and write beams respectively. According to tracking error signals provided by these beams, the disk unit carries out track servo control to trace the target track.
The disk unit also carries out a seek operation by driving the lens actuators. This seek operation is controlled by a processor and carried out with, for example, the write beam at first and then with the erase beam. This raises a problem of increasing an access time in proportion to the number of the beams. This access time must be minimized.
When seeking a target track with the erase beam, the disk unit drives the VCM positioner while counting the number of tracks crossed by the erase beam according to the tracking error signal of the erase beam. After the completion of the seek operation with the erase beam, the unit starts the track servo control. At this time, the unit sets the write beam on the target track without counting the number of tracks crossed by the write beam. Accordingly, the write beam may deviate from the target track. The unit corrects this deviation through a corrective seek operation, which needs time. This means that, to shorten the access time, it is necessary to prevent a deviation of the write beam during the VCM positioner seek operation.